Titanium dioxide (TiO2) has been proven one of the most promising photocatalysts for its intriguing properties like high chemical stability, strong oxidizing power, environment-friendliness, relative economy [H. Li, J. Li, Y. Huo, J. Phys. Chem. B 110 (2006) 1559; M.-S. Wong, H. P. Chou, T.-S. Yang, Thin Solid Films 494 (2006) 244] and so on. Though it is chemically and photochemically stable, at the same time TiO2 photocatalyst is a ultra-violet absorber due to its large energy band gap (3.2 eV for anatase) so that the light utilization efficiency to solar irradiation is very low. Again demerits like low surface area and little adsorbability makes TiO2 an unsuitable candidate for photocatalysis.
To effectively utilize visible light, which represents about 42% energy of the solar spectrum, much attention has been paid to improve the photocatalytic property and visible light response of TiO2 [Y.-M. Lin, Y.-H. Tseng, C.-C. Chen, US Patent 20060034752A1 (2006)]. Among all the methods, a main approach is to dope transition metals into TiO2 [S. Chang, R. Doong, J. Phys. Chem. B 110 (2006) 20808; A. V. Emeline, Y. Furubayashi, X. Zhang, M. Jin, T. Murakami, A. Fujishima, J. Phys. Chem. B 109 (2005) 24441; J. Zhou, Y. Zhang, X. S. Zhao, A. K. Ray, Ind. Eng. Chem. Res. 45 (2006) 3503; H. Liu, T. Peng, D. Ke, Z. Peng, C. Yan, Mater. Chem. Phys. 104 (2007) 377; H. M. Yang, R. R. Shi, K. Zhang, Y. H. Hua, A. D. Tang, X. W. Li, J. Alloys Compd. 398 (2005) 200]. But the non metal doping trend, first discovered by Sato in 1986 and rekindled by Asahi et al. in 2001, seems to be more successful than the transition metal doping [R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269; T. Umebayashi, T. Yamaki, H. Itoh, K. Asai, Appl. Phys. Lett. 81 (2002) 454; S. Sato, Chem. Phys. Lett. 123 (1986) 126]. It is because the metal ion doped TiO2 suffers from thermal instability and the increase of carrier-recombination centres [M.-C. Yang, T.-S. Yang, M.-S. Wong, Thin Solid Films 469-470 (2004) 1]. Therefore non-metal doping into TiO2 may be more appropriate for the extension of its photocatalytic activity into the visible region because their impurity states are near the valence band edge, and their role as recombination centres might be minimized as compared to metal cation doping [D. Chen, Z. Jiang, J. Geng, Q. Wang, D. Yang, Ind. Eng. Chem. Res. 46 (2007) 2741]. Meanwhile, codoped titania with multiple non-metal elements has attracted more attention, such as N,S-codoped TiO2 as a decent visible light photocatalyst [J. Yu, M. Zhou, B. Cheng, X. Zhao, J. Mol. Catal. A: Chem. 246 (2006) 176].
Although the multiple non-metal doping can narrow the band gap of TiO2 photocatalysts and improve the utilization of the solar spectrum, the surface area and adsorbability of the catalyst cannot be increased in this process. Therefore to overcome these difficulties some more modifications to the N,S-codoped TiO2 is required.
For many years clay minerals have been the focus of intensive research due to their ability to intercalate various organic, organometallic or inorganic species into their interlamellar spaces and due to their catalytic properties. Montmorillonite K10 clay is a ubiquitous, inexpensive and non-toxic powder having a high cation exchange capacity (CEC), swelling and intercalation property. These profitable features of the cationic clay make it useful in the move towards establishing environmentally friendly catalysts [A. Vaccari, Catalysis Today 41 (1998) 53]. Due to the features like high CEC and expandable interlayer space, it can accommodate large inorganic metal hydroxycations that are oligomeric and formed by hydrolysis of metal oxides or salts [C. F. Baes, R. E. Meisner, The Hydrolysis of Cations, Wiley, N.Y., 1976].
There are inventions relating to this type of intercalation [J. R. McCauley, U.S. Pat. No. 4,980,047 (1990)]. After calcination the metal hydroxy cations are decomposed into oxide pillars and the materials as a whole get converted to pillared clays. In pillared clays, the oxide pillars keep the clay layers apart and create interlayer and interpillar spaces, thereby exposing the internal surfaces of the clay layer.
Baes and Meisner had reported that any metal oxide or salt that forms polynuclear species upon hydrolysis can be inserted as pillars. Titanium is known to form polymeric species in solution for quite some time [B. I. Nabivanets, L.N. Kudritskaya, Russ. J. Inorg. Chem. 12 (1967) 616; H. Einaga, J. Chem. Soc. Dalton Trans. 12 (1979) 1917]. A number of literatures were reported regarding the various methods to create titania complexes suitable for pillaring processes [J. Sterte, Clays and Clay Minerals 34 (1986) 658; A. Bernier, L. F. Admaiai, P. Grange, Appl. Catalysis 77 (1991) 269; R. T. Yang, J. P. Chen, E. S. Kikkinides, L. S. Cheng, J. E. Cichanowicz, Ind. Eng. Chem. Res. 31 (1992) 1440; N. N. Binitha, S. Sugunan, Micropor. and Mesopor. Mater. 93 (2006) 82; E. Dvininov, E. Popovici, R. Pode, L. Cocheci, P. Barvinschi, V. Nica, J. Hazard. Mater. 167 (2009) 1050; Y. Kitayama, T. Kodama, M. Abe, H. Shimotsuma, Y. Matsuda, J. Porous Mater. 5 (1998) 121; T. Kaneko, H. Shimotsuma, M. Kajikawa, J. Porous Mater. 8 (2001) 295].
Tris-(2, 2′-bipyridyl) ruthenium (II) complex is one of the molecule studied most extensively because of its unique combination of chemical stability, luminescence etc. It is well known for its property as a good photo sensitizer in the water splitting system to produce hydrogen. Various researchers have reported the synthesis of intercalation compounds using this complex in inorganic layered ion-exchanger matrices possessing a non rigid structure such as clay [R. A. DellaGuardia, J. K. Thomas, J. Phys. Chem. 87 (1983) 990; R.A. Schoonheydt, P. De Pauw, D. Vliers, F. C. De Schrijver, J. Phys. Chem. 88 (1984) 5113; V. Joshi, D. Kothar, P. K. Ghosh, J. Am. Chem. Soc. 108 (1986) 4650].
A number of literatures have already been cited regarding the improvement of visible light response of titania by non-metal doping along with pillaring it in clay matrix to improve its surface area as well as adsorbability and applying the modified catalyst in photocatalytic degradation of organic pollutants [G. Zhang, X. Ding, Y. Hu, B. Huang, X. Zhang, X. Qin, J. Zhou, J. Xie, J. Phys. Chem. C 112 (2008) 17994; Z. An-ning, C. You-mei, Y. Zhanjiang, J. of Coal Sci. & Eng.(China) 14 (2008) 517]. But till date neither any report has been published or patented on ruthenium bipyridyl complex intercalated N-doped or N,S-codoped titania pillared montmorillonite.
Thus the scientific literature for the first time discloses a Ruthenium bipyridyl complex intercalated N-doped or N,S-codoped titania pillared montmorillonite, acting as a proficient multifunctional catalyst in various light driven redox reactions. Every reaction involves two reaction pathways that occur under visible light irradiation. One is ascribed to complex photosensitization and the other to the band gap narrowing by only N or N and S doping.